Scallop lens Ω-crystallin (ALDH1A9): A novel tetrameric aldehyde dehydrogenase

Scallop eye lens Ω-crystallin is an inactive aldehyde dehydrogenase (ALDH1A9) related to cytoplasmic ALDH1A1 and mitochondrial ALDH2 that migrates by gel filtration chromatography as a homodimer. Because mammalian ALDH1A1 and ALDH2 are homotetramers, we investigated the native molecular mass of scallop Ω-crystallin by multi-angle laser light scattering. The results indicate that the scallop Ω-crystallin is a tetrameric, not a dimeric protein. Moreover, phylogenetic tree analysis shows that scallop Ω-crystallin clusters with the mitochondrial ALDH2 and ALDH1B1 rather than the cytoplasmic ALDH1A, yet it lacks the mitochondrial N-terminal leader sequence characteristic of the mitochondrial ALDHs. The mitochondrial grouping, enzymatic inactivity, and anomalous gel filtration behavior make scallop cytoplasmic Ω-crystallin an interesting protein for structural studies of evolutionary adaptations to become an enzyme-crystallin.